Occasionally I write sentences containing subparts with extra information. I never know how to block off this information from the rest of the sentence. Three ways I have seen are dashes, commas, and parentheses. For example:

When Bilbo sat down for a meal — his third of the day — he noticed the table was broken.

When Bilbo sat down for a meal, his third of the day, he noticed the table was broken.

When Bilbo sat down for a meal (his third of the day) he noticed the table was broken.

Your questions would be better asked on the English Stack Exchange which deals with issues of English syntax. This site is more for authors and their higher-level writing issues. I have to close this as off topic, but feel free to bring your question to english.stackexchange.com.
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Robert Cartaino♦Feb 5 '11 at 18:50

I think I agree with @Jae I believe all three are technically correct and it would be a matter of style.
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Ralph GallagherFeb 5 '11 at 23:08

1

Agree it's style not grammar; if I had the rep, I'd vote to reopen. From a style standpoint, all three are grammatically correct; to my editorial eye, separating with em-dashes (not hyphens) is more emphatic, with commas is sort of level tone of voice, and parens is sotto voce, a whispered aside.
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Charlie MartinFeb 6 '11 at 1:21

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reopening, I agree this is not strictly a matter of what is correct (technically they ALL are) but what flows best in your writing.
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Jeff Atwood♦Feb 6 '11 at 6:42

4 Answers
4

They are all grammatically correct (as far as I'm aware), but I think they give different feelings.

Parentheses feel like an aside to the audience, like a weaker form of footnotes. I use them a lot in technical writing — often to indicate bits that are optional or additional information/explanation, but that aren't necessary to read — and in more informal writing (like this post) in a similar sense. They tend to be either quite short, or very long (sometimes multiple sentences, in which case they might be better as footnotes). Just don't forget to close the parenthesis at the end — it sticks out if you don't. I don't think they get used very often in fiction writing, possibly because they break the fourth wall and pull the reader out of the story. It's a case of me, the author, speaking directly to you, the reader.

Dashes feel like more substantial interruptions. I think I use them for asides that are less optional (such as the one in the previous paragraph). Sections separated by dashes can hold less that parenthesis, usually no more than a sentence-worth. I've seen people put sentence breaks inside such sections, but it really sticks out (in a bad way). Again, I don't think dashes get used often in fictional writing, though possibly more than parentheses.

Commas (in this context) are the weakest of the three. They can "hold" the least amount of content, but they are also the least noticeable (which is probably why they can contain the fewest words). As such, they don't stand out to the reader — they may not even be noticed consciously — but they do inform the phrasing of the writing.

I don't normally consciously decide between which of the three I use, it's whichever feels right. It also gets a bit more complicated with dashes as they can both be used in other situations, often not as a matching pair (such as the comma in this sentence).

Looking at your three examples, I would say the following:

The first one (dashes) emphasises that it's his third meal of the day, making it at least as important an idea as the broken table.

The second one (commas) downplays the idea, making the broken table more important, just leaving us with an impression about the eating. This is the most subtle of the three.

In the third one (parentheses), the narrator is speaking directly to the reader and is making an out-of-character remark about Bilbo's eating habits. This is the most blatant of the three.

(I would also suggest that he "noticed that the table was broken", but that may just be being picky or a US/UK difference. And I also apologise if this post has rather too many commas, dashes and parentheses.)

Excellent answer, +1. But please use em dashes there — (Alt + 0151). It is also important to note the em dash can be used as a interruption or to mark a unfinished part of a sentence: "Oh look, a cute kitt— WHAT the hell!?". I prefer them over ellipsis … (Alt + 0133), which was suggested in another answer.
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srcspiderFeb 6 '11 at 16:14

I believe question marks exclamation points can be useful within dash-enclosed text as they do not necessarily indicate the end of a sentence.
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Paul A. ClaytonMay 2 '14 at 12:29

@Rob H I agree the additional "that" is preferable to the original wording, to eliminate a possible flicker of ambiguity as to how "the table" functions in the sentence. It's not necessary though, depending on the context and density of the surrounding language. It can be a stylistic thing. I might sacrifice the additional "that" if it made a significant rhythmic difference, or if there were another, more important "that" in the vicinity, and they'd called attention to each other, or made things read too slow or long. Excessive "that" removal, even with optional ones, can hurt clarity.
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NicoleApr 5 at 3:12

I tend to use Emdashes when I'm giving the flow a pause, making the reader stop for a moment and let the words sink in. Commas on the other hand are used to breakup a thought in to segments but still linked.

Parenthesis I consider a tool for the narrator speaking to the reader directly.

You also missed ellipsis, for when you just leave a sentence hanging there, perhaps unfinished.

None of these are correct or incorrect, but my optimal way to say it would be without parentheses or dashes; the former are hard to use well, and the latter are overused in modern fiction.

When Bilbo sat down for his third meal of the day, he noticed the table was broken.

Parentheses can be enormously effective at conveying short snippets of background information, and in my opinion they will, in specific situations, break the flow of the writing less than dashes will. Although I freely admit that seeing text surrounding this example quote could change my answer.